On Mon 2023-08-14 10:42:26, David Laight wrote: > From: Kees Cook > > Sent: 11 August 2023 06:46 > > > > If an output buffer size exceeded U16_MAX, the min_t(u16, ...) cast in > > copy_data() was causing writes to truncate. This manifested as output > > bytes being skipped, seen as %NUL bytes in pstore dumps when the available > > record size was larger than 65536. Fix the cast to no longer truncate > > the calculation. > > > ... > > diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c > > index 2dc4d5a1f1ff..fde338606ce8 100644 > > --- a/kernel/printk/printk_ringbuffer.c > > +++ b/kernel/printk/printk_ringbuffer.c > > @@ -1735,7 +1735,7 @@ static bool copy_data(struct prb_data_ring *data_ring, > > if (!buf || !buf_size) > > return true; > > > > - data_size = min_t(u16, buf_size, len); > > + data_size = min_t(unsigned int, buf_size, len); > > I'd noticed that during one of my test compiles while looking > at making min() less fussy. > > A better fix would be: > data_size = min(buf_size + 0u, len); This looks like a magic to me. The types are: unsigned int data_size; unsigned int buf_size; u16 len I would naively expect that data_size = min(buf_size, len); would do the right job and expand @len to "unsigned int". I do not remember why "min_t" was used. Was it an optimization? Did we miss the problem with casting "u32" down to "u16"? I tried to read the discussion at https://lore.kernel.org/lkml/b6a49ed73aba427ca8bb433763fa94e9@xxxxxxxxxxxxxxxx/ but it is more about "signed" vs. "unsigned" problem. Maybe it is more complicated that I expected. > Or put an ack on my patch 3/5 to minmax.h and then min(buf_size, len) > will be fine (because both arguments are unsigned). Do you mean https://lore.kernel.org/lkml/6dc20ac7cb6f4570a0160f076e8362e3@xxxxxxxxxxxxxxxx/ ? It seems to be just indentation cleanup. Best Regards, Petr PS: I have already pushed the patch because it looked reasonable and got testing. I have to admit that I am probably in a pre-vacation hurry mode.
